A fix for PR21176.

 
DW_OP_const <const> doesn't describe a constant value, but a value at a constant address. 
The proper way to describe a constant value is DW_OP_constu <const>, DW_OP_stack_value. 
Added DW_OP_stack_value to the stack. 

Marked incorrect-variable-debugloc1.ll to xfail for PowerPC64, while the the failure (PR21881) 
is being investigated. 

llvm-svn: 224098

1 files changed, 12 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp b/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
index 230ea465182..b7e9d656e90 100644
--- a/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
@@ -1722,6 +1722,18 @@ void DwarfDebug::emitDebugLocValue(ByteStreamer &Streamer,
       Streamer.EmitInt8(dwarf::DW_OP_constu, "DW_OP_constu");
       Streamer.EmitULEB128(Value.getInt());
     }
+    // The proper way to describe a constant value is 
+    // DW_OP_constu <const>, DW_OP_stack_value. 
+    // Unfortunately, DW_OP_stack_value was not available until DWARF-4,
+    // so we will continue to generate DW_OP_constu <const> for DWARF-2
+    // and DWARF-3. Technically, this is incorrect since DW_OP_const <const>
+    // actually describes a value at a constant addess, not a constant value. 
+    // However, in the past there was no better way  to describe a constant 
+    // value, so the producers and consumers started to rely on heuristics 
+    // to disambiguate the value vs. location status of the expression. 
+    // See PR21176 for more details.
+    if (getDwarfVersion() >= 4)
+      Streamer.EmitInt8(dwarf::DW_OP_stack_value, "DW_OP_stack_value");
   } else if (Value.isLocation()) {
     MachineLocation Loc = Value.getLoc();
     DIExpression Expr = Value.getExpression();